1. Field of the Invention
This invention relates to illumination devices comprising a plurality of light emitting diodes (LEDs) and, more particularly, to illumination devices and methods for calibrating and compensating individual LEDs in the illumination device, so as to obtain a desired luminous flux and chromaticity over time as the LEDs age.
2. Description of the Relevant Art
The following descriptions and examples are provided as background only and are intended to reveal information that is believed to be of possible relevance to the present invention. No admission is necessarily intended, or should be construed, that any of the following information constitutes prior art impacting the patentable character of the subject matter claimed herein.
Lamps and displays using LEDs (light emitting diodes) for illumination are becoming increasingly popular in many different markets. LEDs provide a number of advantages over traditional light sources, such as incandescent and fluorescent light bulbs, including low power consumption, long lifetime, no hazardous materials, and additional specific advantages for different applications. When used for general illumination, LEDs provide the opportunity to adjust the color (e.g., from white, to blue, to green, etc.) or the color temperature (e.g., from “warm white” to “cool white”) to produce different lighting effects.
Although LEDs have many advantages over conventional light sources, one disadvantage of LEDs is that their output characteristics (e.g., luminous flux and chromaticity) vary over changes in drive current, temperature and over time as the LEDs age. These effects are particularly evident in multi-colored LED illumination devices, which combine a number of differently colored emission LEDs into a single package.
An example of a multi-colored LED illumination device is one in which two or more different colors of LEDs are combined within the same package to produce white or near-white light. There are many different types of white light lamps on the market, some of which combine red, green and blue (RGB) LEDs, red, green, blue and yellow (RGBY) LEDs, phosphor-converted white and red (WR) LEDs, RGBW LEDs, etc. By combining different colors of LEDs within the same package, and driving the differently colored LEDs with different drive currents, these lamps may be configured to generate white or near-white light within a wide gamut of color points or correlated color temperatures (CCTs) ranging from “warm white” (e.g., roughly 2600K-3700K), to “neutral white” (e.g., 3700K-5000K) to “cool white” (e.g., 5000K-8300K). Some multi-colored LED illumination devices also enable the brightness and/or color of the illumination to be changed to a particular set point. These tunable illumination devices should all produce the same color and color rendering index (CRI) when set to a particular dimming level and chromaticity setting (or color set point) on a standardized chromacity diagram.
A chromaticity diagram maps the gamut of colors the human eye can perceive in terms of chromacity coordinates and spectral wavelengths. The spectral wavelengths of all saturated colors are distributed around the edge of an outlined space (called the “gamut” of human vision), which encompasses all of the hues perceived by the human eye. The curved edge of the gamut is called the spectral locus and corresponds to monochromatic light, with each point representing a pure hue of a single wavelength. The straight edge on the lower part of the gamut is called the line of purples. These colors, although they are on the border of the gamut, have no counterpart in monochromatic light. Less saturated colors appear in the interior of the figure, with white and near-white colors near the center.
In the 1931 CIE Chromaticity Diagram shown in FIG. 1, colors within the gamut of human vision are mapped in terms of chromaticity coordinates (x, y). For example, a red (R) LED with a peak wavelength of 625 nm may have a chromaticity coordinate of (0.69, 0.31), a green (G) LED with a peak wavelength of 528 nm may have a chromaticity coordinate of (0.18, 0.73), and a blue (B) LED with a peak wavelength of 460 nm may have a chromaticity coordinate of (0.14, 0.04). The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation, E(λ)=Aλ−5/(e(B/T)−1). Color points that lie on or near the blackbody locus provide a range of white or near-white light with color temperatures ranging between approximately 2500K and 10,000K. These color points are typically achieved by mixing light from two or more differently colored LEDs. For example, light emitted from the RGB LEDs shown in FIG. 1 may be mixed to produce a substantially white light with a color temperature in the range of about 2500K to about 5000K. Although an illumination device is typically configured to produce a range of white or near-white color temperatures arranged along the blackbody curve (e.g., about 2500K to 5000K), some illumination devices may be configured to produce any color within the color gamut (triangle) formed by the individual LEDs (e.g., RGB). The chromaticity coordinates of the combined light, e.g., (0.437, 0.404) for 3000K white light, define the target chromaticity or color set point at which the device is intended to operate.
In practice, the luminous flux (i.e., lumen output) and chromaticity produced by prior art illumination devices often differs from the target settings, due to changes in drive current, temperature and over time as the LEDs age. In some devices, the drive current supplied to one or more of the emission LEDs may be adjusted to change the dimming level and/or color point setting of the illumination device. For example, the drive currents supplied to all emission LEDs may be increased to increase the lumen output of the illumination device. In another example, the color point setting of the illumination device may be changed by altering the drive currents supplied to one or more of the emission LEDs. Specifically, an illumination device comprising RGB LEDs may be configured to produce “warmer” white light by increasing the drive current supplied to the red LEDs and decreasing the drive currents supplied to the blue and/or green LEDs.
In addition to affecting changes in the lumen output and/or color point, adjusting the drive current supplied to a given LED inherently affects the junction temperature of that LED. As expected, higher drive currents result in higher junction temperatures (and vice versa). When the junction temperature of an LED increases, the lumen output of the LED generally decreases. For some colors of LEDs (e.g., white, blue and green LEDs), the relationship between luminous flux and junction temperature is relatively linear, while for other colors (e.g., red, orange and especially yellow) the relationship is significantly non-linear.
In addition to luminous flux, the chromaticity of an LED also changes with temperature, due to shifts in the dominant wavelength (for both phosphor converted and non-phosphor converted LEDs) and changes in the phosphor efficiency (for phosphor converted LEDs). In general, the peak emission wavelength of green LEDs tends to decrease with increasing temperature, while the peak emission wavelength of red and blue LEDs tends to increase with increasing temperature. While the change in chromacity is relatively linear with temperature for most colors, red and yellow LEDs tend to exhibit a more significant non-linear change.
While some prior art devices do perform some level of temperature compensation, they fail to provide accurate results by failing to recognize that temperature affects the lumen output and chromaticity of different colors of LEDs differently. Moreover, these prior art devices often fail to account for changes in lumen output and chromaticity that occur gradually over time as the LEDs age.
As LEDs age, the lumen output from both phosphor converted and non-phosphor converted LEDs, and the chromaticity of phosphor converted LEDs, also changes. Early on in life, the luminous flux can either increase (get brighter) or decrease (get dimmer), while late in life, the luminous flux generally decreases. FIGS. 2-3 demonstrate how the lumen output of an exemplary emission LED changes over temperature (e.g., 55° C., 85° C. and 105° C.) and over time (e.g., 1,000 to 100,000 hours) for two different fixed drive currents (e.g., 0.7 A in FIG. 2 and 1.0 A in FIG. 3). As expected, lumen output decreases faster over time when the LED is subjected to higher drive currents and higher temperatures.
As a phosphor converted LED ages, the phosphor becomes less efficient and the amount of blue light that passes through the phosphor increases. This decrease in phosphor efficiency causes the overall color produced by the phosphor converted LED to appear “cooler” over time. Although the dominant wavelength and chromaticity of a non-phosphor converted LED (e.g., a red, green, blue, etc. LED) does not change over time, the lumen output decreases over time as the LED ages (see, FIGS. 2-3), which in effect causes the chromaticity or color set point of a multi-colored LED illumination device to change over time. Without accounting for LED aging affects, prior art devices cannot maintain a desired luminous flux and a desired chromaticity for an LED illumination device over the lifetime of the illumination device.
A need remains for improved illumination devices and methods for calibrating and compensating individual LEDs within an LED illumination device, so as to accurately maintain a desired luminous flux and a desired chromaticity for the illumination device over changes in temperature, changes in drive current and over and time as the LEDs age. This need is particularly warranted in multi-color LED illumination devices, since different colors of LEDs are affected differently by temperature and age, and in tunable illumination devices that enable the target dimming level and/or the target chromaticity setting to be changed by adjusting the drive currents supplied to one or more of the LEDs, since changes in drive current inherently affect the lumen output, color and temperature of the illumination device.